The microstructure of melt-spun Au83.3Co16.7 and Au71.6Co28.4 ribbons is investigated by means of analytical and high-resolution electron microscopy. The as-quenched ribbons contain a multilayered lamellar eutectic within the grains and Co precipitates at grain boundaries. Cobalt-rich lamellae are found with growing direction (110) and an average width of 7.5 nm. The thickness of the Co-lamella parallel to [001] is larger in the sample with the higher Co-content. Annealing at 480-degrees-C dissolves the eutectic and large Co particles are formed at grain boundaries and within the grains deteriorating the magnetic properties.